Quantum-counting x-ray detectors are used in many imaging applications. X-ray detectors of this type are used for example in computed tomography systems for medical imaging, to produce a tomographic x-ray image of an examination region of a patient. With a quantum-counting radiation detector different unwanted effects occur as a function of the size of the individual detector elements, hereafter also referred to as pixels, resulting in conflicting requirements when selecting the pixel size of the detector. The smallest possible pixel size is advantageous for the high flux response, to reduce the probability of the pile-up effect and to utilize the small pixel effect. However it is advantageous to select the largest possible pixels for good energy resolution, in order to reduce the unwanted effects of charge sharing and the K-escape on the energy registered in a pixel. The last-mentioned two effects have a greater influence on the measurement result with smaller pixels. Moreover these unwanted effects also make an additional noise contribution due to their statistical occurrence. Pixel size selection also ultimately determines the achievable spatial resolution of the x-ray detector.
In many applications these contradictory requirements require compromises when selecting the size of the detector elements or the pixel size, eroding the advantage of a quantum-counting detector compared with an energy-integrating detector. Thus when a quantum-counting detector is used in a clinical computed tomography system, the required x-ray flux necessarily requires the selection of a pixel size of only 100 μm to 300 μm, as otherwise the pixel would be forced into paralysis at full x-ray flux and would no longer supply usable data. However with such small pixel sizes, when quantum-counting detectors, particularly those made of CdTe or CZT(CdZnTe), are used, the loss of energy resolution and the additional noise contribution due to charge sharing and K-escape are already significant.
A quantum-counting x-ray detector is known from WO 2009/042827, wherein larger detector units are formed from the individual detector elements, by adding or otherwise combining the count results of a number of adjacent detector elements. However this procedure brings with it no advantage in respect of the unfavorable effects of K-escape and charge sharing, as the resulting error can no longer be determined and corrected from the count results alone.
US 2009/0080601 describes a quantum-counting x-ray detector, wherein the detector elements are combined dynamically to form larger detector units, by bringing the detector elements into electrical contact with one another by way of a matrix of switches and forwarding the resulting signal to a common comparator. This method requires an estimation of the expected x-ray flux before measurement, with the result that its suitability is limited in many applications. The abrupt switching between the different sizes of the resulting detector units also gives rise to major problems, as the requirement of a continuously differentiatable connection condition for the signals resulting from the different pixel sizes cannot be satisfied in a simple manner.
A quantum-counting x-ray detector is known from DE 10 2004 048 962, wherein the charge pulses of adjacent detector elements are added together in an analog manner when they are identified by a coincidence circuit as being associated with a photon event.
Finally WO 2004/008488 illustrates a detector according to the preamble of claim 1, wherein the signals from a number of detector elements are summed in an analog manner and then forwarded to a common discriminator and counter, to utilize the small pixel effect.
The two last-mentioned charge summing methods cannot however improve the response of the detector with high flux. Their paralysis response corresponds approximately to that of a detector with pixels, the area of which corresponds to the overall area of the pixels combined respectively to form a larger detector unit. A charge summing method which combines for example the signals from four pixels demonstrates approximately the same paralysis response as an individual pixel of four times the basis area, so detector paralysis starts at approximately a quarter of the quantum flux.